The present invention relates to a tracking method and apparatus used in a helical scan magnetic recording/reproducing system, and more particularly to such a tracking method and apparatus using an envelope detection signal of a reproduction signal.
In a helical scan magnetic recording/reproducing system, a tracking control is performed such that at the time of reproduction, a magnetic head traces recording tracks obliquely formed on a magnetic tape. This tracking control will now be explained with reference to FIG. 9.
In FIG. 9, reference numeral 52 denotes a magnetic tape which is transported at a speed Vt in a direction indicated by the arrow. Recording tracks j-2 to j+2 are recorded for each field in the order of j-2, j-1, - - - , j+2. Numeral 21 denotes a magnetic head which scans at a speed Vd in a direction indicated by the arrow. Symbols a, b and c represent three different positions of the magnetic head 21 relative to the recording track j in a horizontal direction. The position a represents the case where the magnetic head scans the center of the track, that is, a condition in which the tracking is correct. The position b represents the case where the tracking has a deviation to the succeeding track side, and the position c represents the case where the tracking has a deviation to the preceding track side. In a tracking control, the transport speed of the tape is controlled so that the magnetic head always takes the condition a. If the magnetic head takes the condition b, the magnetic tape is decelerated. If the magnetic head takes the condition c, the magnetic tape is accelerated. Though the control of the transport speed of the tape is made by the control of a rotation speed of a capstan motor, the acceleration/deceleration control is equivalent to the control of a rotation phase which is the integrated value of the rotation speed. Therefore, the following explanation will be made referring to such a control as phase control.
Known tracking systems are categorized into several types of tracking systems with respect to the method for detection of a tracking deviation. A known typical tracking system includes a control signal system (hereinafter referred to as a CTL system) which is employed in a VHS system or the like, and an ATF system which uses a pilot signal and is employed in an 8 mm VTR or the like. In the CTL system, a tracking control is performed by controlling the transport speed of a magnetic tape so that a control signal (or CTL signal) recorded at a frame period in a longitudinal direction of the tape and the phase of rotation of a cylinder on which a magnetic head is mounted have a predetermined phase relation therebetween. Also, in an example of the ATF system, for example, in an 8 mm VTR, four kinds of pilot signals having different frequencies are cyclically recorded for successive fields at the time of recording and a tracking control at the time of reproduction is performed by detecting the recorded pilot signals to control the transport speed of a tape so that the levels of pilot signals reproduced from two tracks on opposite sides of a main track to be scanned become equal to each other.
Though each of the CTL system and the ATF system uses a signal for tracking control, there is also known a tracking control system in which no dedicated signal for tracking is used and the level of a reproduced video signal is detected to control a tape speed so that the detected level becomes a maximum. For example, JP-A-58-1843 has disclosed a tracking apparatus of a CTL system in which means for delaying a reproduced CTL signal to shift a tracking phase and means for making a comparison in level of the envelope detection output of a reproduced video signal are provided for performing a so-called "mountain climbing method" (or automatic tracking control). In the automatic tracking control, the increase of the envelope level by the shift of the tracking phase is followed by the application of a phase shift in the same direction and the decrease of the envelope level is followed by the inversion of the direction of phase shift, so that a maximum envelope level is attained. Thereby, a tracking deviation which may be caused by, for example, a deviation in attachment position of a CTL head between a recording apparatus and a reproducing apparatus is corrected.
FIG. 10A schematically shows the operation of the automatic tracking control in which an envelope detection signal is used. When a tracking phase is shifted from a position denoted by "0" in FIG. 10A by .DELTA.x in a positive (+) direction, the shift to a position denoted by "1" is made. Thereby, an envelope level increases. Then, the next phase shift is made in the same direction. When the shift is made to a position denoted by "4", the envelope level decreases. Then, the direction of phase shift is inverted so that the shift to a position denoted by "5" is made. Subsequently, the phase shift steps from the position "5" to a position "7" are performed, thereby performing a tracking control so that the envelope level takes a value in the vicinity of a maximum value.
The following publications other than JP-A-58-1843 are relevant to tracking control techniques.
U.S. Pat. No. 4,613,914 has disclosed a technique in which a high-precision tracking is performed with a shortened search time by performing a fine search after first performing a coarse search of one track pitch to obtain a range of tracking phase in which a better tracking condition is provided.
U.S. Pat. No. 4,777,543 has disclosed a technique in which automatic tracking is stabilized by detecting a plurality of points on a reproduced envelope in one period to take a minimum value of those points as a sample value of a tracking condition and controlling a tracking phase so that the sample value becomes a maximum.
JP-A-55-12563 has disclosed a technique in which the phase of a capstan servo is changed to detect a change in envelope level of a reproduced signal before and after the phase change and a control is performed so that the envelope level becomes a maximum.
JP-A-2-312048 has disclosed a technique in which first tracking means for fluctuating the speed of a capstan motor at a predetermined rate to perform synchronous detection and second tracking means for performing a tracking based on a reproduced envelope signal are provided, whereby a tracking control is performed by the first means in a start-up or build-up period of time after a mode transition and is thereafter performed by the second means. This publication has also disclosed correcting a speed offset by integrating a phase error signal produced by the first means.